finding 2.2 : key-finding-2-2

Aerosols caused by human activity play a profound and complex role in the climate system through radiative effects in the atmosphere and on snow and ice surfaces and through effects on cloud formation and properties. The combined forcing of aerosol–radiation and aerosol–cloud interactions is negative (cooling) over the industrial era (high confidence), offsetting a substantial part of greenhouse gas forcing, which is currently the predominant human contribution. The magnitude of this offset, globally averaged, has declined in recent decades, despite increasing trends in aerosol emissions or abundances in some regions. (Medium to high confidence)

This finding is from chapter 2 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: This key finding is consistent with the findings of IPCC AR56c7c285c-8606-41fe-bf93-100d80f1d17a that aerosols constitute a negative radiative forcing. While significant uncertainty remains in the quantification of aerosol ERF, we assess with high confidence that aerosols offset about half of the positive forcing by anthropogenic CO2 and about a third of the forcing by all well-mixed anthropogenic GHGs. The fraction of GHG forcing that is offset by aerosols has been decreasing over recent decades, as aerosol forcing has leveled off while GHG forcing continues to increase.

Description of evidence base: The Key Finding and supporting text summarize extensive evidence documented in the climate science literature, including in previous national (NCA3)dd5b893d-4462-4bb3-9205-67b532919566 and internationalf03117be-ccfe-4f88-b70a-ffd4351b8190 assessments. Aerosols affect Earth’s albedo by directly interacting with solar radiation (scattering and absorbing sunlight) and by affecting cloud properties (albedo and lifetime).

Fundamental physical principles show how atmospheric aerosols scatter and absorb sunlight (aerosol–radiation interaction), and thereby directly reduce incoming solar radiation reaching the surface. Extensive in situ and remote sensing data are used to measure emission of aerosols and aerosol precursors from specific source types, the concentrations of aerosols in the atmosphere, aerosol microphysical and optical properties, and, via remote sensing, their direct impacts on radiative fluxes. Atmospheric models used to calculate aerosol forcings are constrained by these observations (see Key Finding 1).

In addition to their direct impact on radiative fluxes, aerosols also act as cloud condensation nuclei. Aerosol–cloud interactions are more complex, with a strong theoretical basis supported by observational evidence. Multiple observational and modeling studies have concluded that increasing the number of aerosols in the atmosphere increases cloud albedo and lifetime, adding to the negative forcing (aerosol–cloud microphysical interactions) (e.g., Twohy 2005;def9f038-e2ec-4e06-8c53-ad2dba2644c0 Lohmann and Feichter 2005;29287dd0-b0df-48e6-b440-1a2e8ecf95db Quaas et al. 2009;1059441a-bc93-46b7-a298-e9e11daed482 Rosenfeld et al. 2014799a97d2-cc70-4fa0-8ffe-5e153de471a3). Particles that absorb sunlight increase atmospheric heating; if they are sufficiently absorbing, the net effect of scattering plus absorption is a positive radiative forcing. Only a few source types (for example, from diesel engines) produce aerosols that are sufficiently absorbing that they have a positive radiative forcing.c024a923-aedd-4e72-8555-3a37ccc41e14 Modeling studies, combined with observational inputs, have investigated the thermodynamic response to aerosol absorption in the atmosphere. Averaging over aerosol locations relative to the clouds and other factors, the resulting changes in cloud properties represent a negative forcing, offsetting approximately 15% of the positive radiative forcing from heating by absorbing aerosols (specifically, black carbon).c024a923-aedd-4e72-8555-3a37ccc41e14

Modeling and observational evidence both show that annually averaged global aerosol ERF increased until the 1980s and since then has flattened or slightly declined,d4333f5a-a657-44d3-a8dd-b9c628336e95 e4d24d55-221f-431c-8f49-731d6de01f7d dbe0e8b3-9c80-4cdd-9385-87af5b0c353f ac44788a-26b4-4d75-bd03-dc74ec2e56be driven by the introduction of stronger air quality regulations (Smith and Bond 201451fb7518-6330-4606-8267-1f5a94fb567f; Fiore et al. 2015b4038a28-b14b-4ae8-b783-0de19e3cffdd). In one recent study,e63f1a17-f6f7-4d2d-9b07-3446052d2cf7 global mean aerosol RF has become less negative since IPCC AR5,6c7c285c-8606-41fe-bf93-100d80f1d17a due to a combination of declining sulfur dioxide emissions (which produce negative RF) and increasing black carbon emissions (which produce positive RF). Within these global trends there are significant regional variations (e.g., Mao et al. 201449942fe8-edc8-49ee-8c42-62d1f552cb92), driven by both changes in aerosol abundance and changes in the relative contributions of primarily light-scattering and light-absorbing aerosols.b4038a28-b14b-4ae8-b783-0de19e3cffdd e63f1a17-f6f7-4d2d-9b07-3446052d2cf7 In Europe and North America, aerosol ERF has significantly declined (become less negative) since the 1980s.97125282-522e-434e-9809-58f09bb62963 e5093ad6-fff0-48b1-863b-51882837f648 b88f12b9-a064-41a2-ac22-f607a0808ffb c93061f6-55c4-4cb7-aeed-4d3fb038988f 0c21112e-35a5-4f00-acc9-89518c87aeef 27a3f365-968e-437c-b605-22ce1bddc2c5 In contrast, observations show significant increases in aerosol abundances over India,b5aaf5bf-8a95-4770-bc35-8374bede2212 c6aed4e2-0b27-4270-9f87-4bf30921e995 and these increases are expected to continue into the near future.5ca4fe4e-4470-4e1a-bf60-ebdb4c5e5919 Several modeling and observational studies point to aerosol ERF for China peaking around 1990,68dff840-d073-4300-bdc6-7600b73a1c68 6d064098-e255-418e-afd2-3985177cf082 d9eb65a0-449a-4b33-9936-ecc789ed814a though in some regions of China aerosol abundances and ERF have continued to increase.d9eb65a0-449a-4b33-9936-ecc789ed814a The suite of scenarios used for future climate projection (i.e., the scenarios shown in Ch. 1: Our Globally Changing Climate, Figure 1.4) includes emissions for aerosols and aerosol precursors. Across this range of scenarios, globally averaged ERF of aerosols is expected to decline (become less negative) in the coming decades,51fb7518-6330-4606-8267-1f5a94fb567f e4d24d55-221f-431c-8f49-731d6de01f7d reducing the current aerosol offset to the increasing RF from GHGs.

New information and remaining uncertainties: Aerosol–cloud interactions are the largest source of uncertainty in both aerosol and total anthropogenic radiative forcing. These include the microphysical effects of aerosols on clouds and changes in clouds that result from the rapid response to absorption of sunlight by aerosols. This finding, consistent across previous assessments (e.g., Forster et al. 2007;f2b357c2-f4ae-4868-a058-e48fbdbb1303 Myhre et al. 20136c7c285c-8606-41fe-bf93-100d80f1d17a), is due to poor understanding of how both natural and anthropogenic aerosol emissions have changed and how changing aerosol concentrations and composition affect cloud properties (albedo and lifetime).9e2542c2-865e-4863-98d1-242b11016592 97e50b82-dfba-40d7-9fab-3e9d3b75b1d5 From a theoretical standpoint, aerosol–cloud interactions are complex, and using observations to isolate the effects of aerosols on clouds is complicated by the fact that other factors (for example, the thermodynamic state of the atmosphere) also strongly influence cloud properties. Further, changes in aerosol properties and the atmospheric thermodynamic state are often correlated and interact in non-linear ways.2c6d403c-80e0-4bd0-bfb7-40440d041974

Assessment of confidence based on evidence: There is very high confidence that aerosol radiative forcing is negative on a global, annually averaged basis, medium confidence in the magnitude of the aerosol RF, high confidence that aerosol ERF is also, on average, negative, and low to medium confidence in the magnitude of aerosol ERF. Lower confidence in the magnitude of aerosol ERF is due to large uncertainties in the effects of aerosols on clouds. Combined, we assess a high level of confidence that aerosol ERF is negative and sufficiently large to be substantially offsetting positive GHG forcing. Improvements in the quantification of emissions, in observations (from both surface-based networks and satellites), and in modeling capability give medium to high confidence in the finding that aerosol forcing trends are decreasing in recent decades.

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